FIG. 1 illustrates a vehicle powertrain. Heavy lines indicate mechanical power flow whereas thin lines indicate flow of transmission fluid. Engine 10 drives torque converter 12 which, in turn, drives gearbox 14. Gearbox 14 may adjust the speed and torque before transmitting the mechanical power to an output shaft. The gear ratio of gearbox 14 is selected by providing pressurized fluid to hydraulically actuated clutches. Pump 16, driven mechanically by engine 10, draws fluid from sump 18. Valve body 20 routes the pressurized fluid to the torque converter and to the clutches within gearbox 14 that establish the desired gear ratio. The fluid also provides lubrication to gearbox 14 and absorbs heat. The fluid then returns to sump 18.
The transmission operates most efficiently when the fluid is at an optimal temperature. When the fluid is too cold, its viscosity is higher increasing parasitic drag. If the fluid gets too hot, the viscosity is too low resulting in increased leakage around the pump and elsewhere. This increased leakage reduces the pressure available from pump 16 reducing the torque capacity of the clutches within gearbox 14. If the fluid temperature remains high for a sufficient period of time, the friction characteristics of the clutches change and shift quality degrades. The temperature of the fluid is controlled by routing the lubrication fluid through cooler 22 and bypass valve 24. The cooler is a heat exchanger with a fluid loop designed to facilitate heat transfer either directly to ambient air, or to an intermediate medium such as liquid coolant. When the fluid temperature is high, lubrication fluid is routed through cooler 22 before entering the gearbox 14. When the fluid temperature is low, on the other hand, bypass valve 24 routes the fluid directly to gearbox 14 bypassing the cooler and thus permitting the fluid to warm up quicker. Note that, although valve body 20 and bypass valve 24 are illustrated in FIG. 1 as distinct components, some embodiments may integrate bypass valve 24 into the valve body.
Some pressure drop is normal as the fluid flows through cooler 22. However, in some conditions, the resistance may be excessive resulting in an unacceptable pressure drop. This can occur, for example, when the fluid in the cooler fluid loop is very cold and therefore has very high viscosity. The fluid within the cooler can be cold even when the fluid circulating within the transmission has warmed up because the bypass valve has been segregating the fluid. In some cases, the resistance is high enough to completely block the flow through the lubrication and cooling circuit of the transmission, risking damage to components of the gearbox.